In industrial coating processes such as those used in automotive manufacturing, efforts are constantly made to reduce energy consumption and costs, as well as atmospheric pollution caused by volatile solvents which are emitted during a painting process. However, it is often difficult to achieve high quality, smooth coating finishes, such as are required in the automotive industry, without using organic solvents in the paint compositions. Solvents improve the flow and leveling of a coating during application to a substrate, thereby improving the coating's final appearance. It is also difficult to provide adequate physical properties without applying multiple coating layers, each having their own cure regimen. In addition to achieving near-flawless appearance, automotive coatings must be durable and chip resistant, historically made possible by using multiple coating layers, each serving its own purpose.
The current state of the art automobile painting process involves electrophoretic application of a paint layer to the bare or treated metal substrate followed by fully curing the so-applied layer. A primer layer, whose purpose is primarily to provide chip resistance, UV opacity, and substrate filling (to smooth surface defects) is then applied, followed again by a full curing cycle. A colored basecoat layer is then applied, generally followed by a heated flash and then application of a final clearcoat layer. These two layers are then co-cured to produce the final coated article. There has been a tendency in the last decade to reduce the paint booth footprint, reduce the number of intermediate bake cycles and hence energy expenditure, reduce the number of coating layers and therefore system complexity, while maintaining the high level of optical quality and appearance of the resulting coated vehicles. The general name given to such modified paint processes is Compact Process.
In order to reduce layers, it is usually the primer layer and its associated oven that is eliminated, and the basecoat composition is then typically designed to incorporate some of the primer properties such as chip resistance and substrate filling. In this case the basecoat is typically applied in two layers with the composition of the first layer being modified to incorporate some heretofore primer-associated properties. After application of the two basecoat layers, a heated flash may be employed to remove some of the solvent and is followed by clearcoat application. The multi-component composite coating composition, or “coating stack”, is then co-cured to provide the final article. In order to provide desired basecoat opacity and protection of the electrocoat layer, the sum of basecoat layer thicknesses is generally greater than the thickness of a basecoat applied over a fully baked conventional primer.
An alternate possibility is known as a 3C1B (3 coat-1 bake) process and involves keeping the primer layer per se, but removing the complete bake after the primer layer. The three layers (primer/basecoat/clearcoat) are applied wet-on-wet-on-wet, with or without heated intermediate flashes between layers, and co-cured in a single cure oven to produce the final article. This process maintains the functionality of the primer layer but removes the cost associated with the primer oven.
In the absence of polymers specifically designed for these Compact Process applications, issues like decreased pop and pinhole resistance due to increased solvent content, increased tendency of the coating stack to mud crack, decreased sag resistance, slumping of the basecoat layer, and/or increased interlayer strike-in, can result in worse optical appearance, poorer color control, and/or poorer process robustness.
Compact Coating systems that provide the desired physical and optical quality over a range of intermediate flash conditions are needed in order to accommodate the different processing parameters of different manufacturers. The system must also be designed to guarantee appearance consistency and quality at different locations on the same vehicle, which may undergo different process conditions during coating. Finally, manufacturers who currently employ heated intermediate flashes are constantly looking to reduce the temperature and time of these steps so they can reduce their energy expense and reduce their line footprint. For these reasons, it is desired to develop resins and coating compositions that provide coating system robustness and coating quality in a Compact Process while reducing process energy requirements.